Author's Note: Basically this is an explanation for why he randomly has a water pistol in his pocket in Fires of Pompeii. Hope you enjoy, and feel free to comment.
Disclaimer: I do not own these characters, they belong to Russell T. Davies.
The Water Before the Flame
"Doctor?"Donna called coming round a bend in a corridor of the TARDIS. "Where ar-" a splash of water met her face, leaving it wet and dripping onto her shirt. "Ohhhhhh!" the harsh growl escaped her throat as a laugh echoed from around the bend. In a hot Celtic temper she continued, "When I get my hands on your scrawny neck... why I oughta!"
"You gotta catch me first." The Doctor's voice came with the sound of running footsteps. She stepped around the corner and nearly stepped on a filled orange water-pistol.
A sinister grin stretched across her face as she took up the watery weapon. "If it's a fight you want it's a fight you'll get." She chased after the sound of running. Occasionally a wet confrontation would occur at the rounding of a corner. It quickly became obvious, however, that the TARDIS was on the Doctor's side. She would be on his heels until he turned a corner , following she would find herself faced with a dead end.
Even so she managed to land quite a lot of good shots on him, and he her. At one point they both ran out of water around the same time. Each immediately rushed to a bathroom, by chance the same one. Meeting at the sink, a squabble occurred over the filling. With a great amount of splashing both pistols eventually got filled.
Ammunition renewed the battle commenced full force. During this round the TARDIS seemed to interfere less. Ambush became a popular strategy. One would sit in wait, pistol at the ready, until the other passed by; at which time a wet confrontation would occur until one or both fled the area.
The battle ended in one last, and rather magineficent, hurrah. Donna had just chased her quarry, the Doctor, into an empty room of cement walls and a gorgiously tiled floor. Standing to the farside of the center the Doctor placed his water pistol into his scientifically enlarged pocket and raised his arms as though in sign of surrender, a supiciously mischevious grin on his face. These unusual actions maybe explained simply by the plan he thought would work. As Donna advanced into the room his little trap was put in motion. However, the third party involved had other ideas. I would prove that the TARDIS would have the last laugh in this escapade. A curtain of water, according to the Doctor's plan meant for Donna alone, fell to engulf both occupant's of the room.
"What the..." the Doctor began, dripping wet, and caught in his own trap.
Realizing his puzzlement an immense amused grin spread across Donna's wet face. "You mean... you weren't... You got caught in your own trick!" As she began laughing uncontrollably he couldn't help to join in, at his own expense. The force of their laughter brought them to floor, rolling in helpless mirth.